Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras, have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. A PD may be classified as class 0 to class 4. A PD of class 1 requires that the PSE supplies at least 4.0 W, a PD of class 2 requires that the PSE supplies at least 7.0 W, and a PD of class 0, 3 or 4 requires at least 15.4 W. Based on the determined class of the PD, the PSE applies the required power to the PD.
When power is supplied to the PD, the IEEE 802.3af standard requires the PSE to check for an overcurrent condition by monitoring its output current with respect to certain current limit thresholds, such as the maximum output current of the PSE at a short circuit condition (ILIM), and the overload current detection range (ICUT). In particular, the PSE should be able to withstand without damage the application of short circuits of any wire to any other wire within a power supply cable, if the magnitude of the current through such a short circuit does not exceed ILIM. Further, an overload condition may be detected when an output current of the PSE exceeds ICUT for a time period exceeding an overload time limit (Tovld) set in the range between 50 ms and 75 ms. To comply with the IEEE 802.3af standard, a value of ILIM must be maintained in the range between 400 mA and 450 mA, while a value of ICUT must be kept at a level which is more than 15.4 W/VPort but less than 400 mA, where VPort is an output voltage of the PSE.
Also, the IEEE 802.3af standard requires the PSE to check for an undercurrent condition to make sure that the PD is drawing the minimum specified current. In particular, in accordance with the Maintain Power Signature (MPS) requirement, the PSE may monitor its output current to remove power from the port if the output current is below the IDLE state current (IMin) for a time period greater than an MPS dropout time limit (TMPDO). The IEEE 802.3af standard requires a value of IMin to be in the range between 5 mA and 10 mA.
Hence, the PSE is required to measure its output current to respond to an overcurrent condition when the output current exceeds the current limit thresholds and to respond to an undercurrent condition when the output current is below a certain minimum value. Usually, the PSE measures its output current by determining a sense voltage across a sense resistor.
For thermal or voltage headroom reasons, it is desirable to make the sense resistor as small as possible, especially if the current has a wide dynamic range. For example, a 0.5Ω resistor may be used to comply with the IEEE 802.3af standard. Because of a low value of the sense resistor, a regular differential amplifier used for measuring the sense voltage and comparing the corresponding current with required threshold levels can cause significant errors due to an offset voltage associated with amplifier circuitry. For example, the offset voltage may be caused by dynamic conditions, such as thermal, light and radiation conditions, by differences in the size of the input stage transistors, by differences in the doping and base diffusion of these transistors, and other circuitry imperfections. Due to the offset voltage, the differential amplifier may produce some signal at its output even when voltages applied to its inputs are the same.
Therefore, it would be desirable to provide a PSE with a current measuring circuit that would compensate for the offset voltage of the amplifier circuitry to produce a correct value at the output of the amplifier.